FIG. 1 illustrates a heterogeneous network wireless communications system 100 including a macro eNB (MeNB) and a micro eNB. In the description of the present invention, the term “heterogeneous network” refers to a network in which a macro eNB 110 and a micro eNB 120 are present together even though the same Radio Access Technology (RAT) is used.
The macro eNB 110 is a general eNode B (base station) of a wireless communication system which has a broad coverage and high transmission power. The macro eNB 110 may also be referred to as a macro cell.
The micro eNB 120 may also be referred to as a micro cell, a pico cell, a femto cell, a home eNB (HeNB), or a relay. As a small-sized version of the macro eNB 110, the micro eNB 120 may independently perform most of the functions of the macro eNB. The micro eNB 120 may be installed (in an overlay manner) in an area covered by the macro eNB or may be installed (in a non-overlay manner) in a shadow area that cannot be covered by the macro eNB. The micro eNB 120 has a narrower coverage and lower transmission power and may accommodate a smaller number of user equipments (UEs), compared to the macro eNB 110.
A UE 131, which is hereinafter referred to as a macro UE (MUE), may be directly served by the macro eNB 110. A UE 132, which is hereinafter referred to as a micro UE or Home UE (HUE), may be served by the micro eNB 120. In some cases, the UE 132 present within the coverage of the micro eNB 120 may be served by the macro eNB 110.
The micro eNB may be classified into two types according to access limitations of the UE. The first type is a Closed Subscriber Group (CSG) micro eNB, and the second type is an Open Access (OA) or Open Subscriber Group (OSG) micro eNB. More specifically, the CSG micro eNB may serve only specific authorized UEs, and the OSG micro eNB may serve all types of UEs without any particular access limitations.